Field of the Invention
The present invention relates to color conversion processing of image data acquired by an image input device.
Description of the Related Art
The spectral characteristics of the color filters of an image input device represented by a digital camera do not generally coincide with human visual spectral characteristics (color-matching functions). The RGB values of image data captured by an image input device are values (to be referred to as device-dependent values hereinafter) depending on the characteristics of the image input device. It is, therefore, necessary to convert the RGB values as device-dependent values into tristimulus values (XYZ values) serving as device-independent color signals or into standard RGB values in an sRGB space, an AdobeRGB space, or the like. For example, there is known a method of converting RGB values into tristimulus values by matrix calculation.
However, in matrix calculation, no high-accuracy conversion is performed especially for a high chroma color, thereby degrading the color conversion accuracy. To cope with this, for example, International Publication No. 2013/101639 discloses a method of performing high-accuracy color conversion processing in the entire visible light range by calculation using a lookup table (LUT) instead of matrix calculation when converting RGB values into XYZ values.
In a technique disclosed in International Publication No. 2013/101639, to create an LUT to be used for color conversion, grid points are arranged in the entire visible light range, and spectral data corresponding to each grid point is generated. After that, RGB values and XYZ values for the spectral data of each grid point are calculated and the correspondence between the RGB values and the XYZ values is recorded in the LUT.
According to International Publication No. 2013/101639, as spectral data corresponding to each grid point, data (to be referred to as rectangular spectral data hereinafter) having spectral characteristics (to be referred to as rectangular spectral characteristics hereinafter) of a rectangular wave shape in which the strength is 0 in a given wavelength range and constant strength is obtained in the remaining wavelength range is used. The rectangular spectral characteristics cannot sufficiently express the spectral characteristics of an existing object. Especially, the spectral characteristics of a low chroma object are largely different from the rectangular spectral characteristics. Therefore, if color conversion is performed by an LUT created using the rectangular spectral data, the conversion accuracy of an object having spectral characteristics different from the rectangular spectral characteristics deteriorates.